19.6 : Physical Properties of Amines

Amines with low molecular weight are usually gaseous at room temperature, while those with high molecular weight are liquid or solids in nature. Usually, low molecular weight amines have a rotten fish-like smell. Diamines typically have a pungent smell. For instance, cadaverine and putrescine, depicted in Figure 1, are two molecules responsible for decaying tissue.

Figure 1. The chemical structures of putrescine and cadaverine.

Amines are polar. Accordingly, primary and secondary amines with N–H bonds enable intermolecular hydrogen bonding interactions. This interaction results in a higher boiling point of amines than alkanes or ethers of comparable molecular weight, which only have weaker interactions such as van der Waals forces or dipole–dipole interactions. On the other hand, alcohols have higher boiling points than amines because nitrogen is less electronegative than oxygen, resulting in weaker intermolecular hydrogen bonding between nitrogen and hydrogen than oxygen and hydrogen. This scheme is captured in Figure 2.

Figure 2. The influence of the functional group on the boiling points of different compounds.

As illustrated in Figure 3, the tertiary amines have lower boiling points than primary or secondary amines. As opposed to primary or secondary amines, the absence of the N–H bond in tertiary amines eliminates the chances of hydrogen bonding.

Figure 3. The influence of hydrogen bonding on the boiling point.

Low molecular weight amines are also soluble in water—the hydrogen bonding between amines and water, as shown in Figure 4, results in this solubility. The water solubility of amines decreases as the molecular weight increases. Generally, amines with fewer than five carbon atoms are water-soluble. Due to the interaction between the electron pair of the nitrogen atom and the hydrogen atom of water, even tertiary amines are soluble in water.

Figure 4. The hydrogen bond between an amine and water.

In general, amines are significantly toxic and can induce poisoning or death if ingested because of their physiological activity. For example, arylamines such as benzidine and β-naphthylamine, presented in Figure 5, are carcinogenic and should be handled cautiously.

Figure 5. Well-known carcinogenic arylamines.

Moreover, skin secretions of the poison dart frog Phyllobates terribilis from South America have been observed to consist of two toxic amine compounds: batrachotoxin and batrachotoxinin A. These amines are so poisonous that even an amount as low as 200 µg is enough to induce irreversible cardiac arrest in human beings.